Many medical devices are now designed to be implantable including pacemakers, defibrillators, circulatory assist devices, cardiac replacement devices such as artificial hearts, cochlear implants, neuromuscular simulators, biosensors, and the like. Since almost all of the active devices (i.e., those that perform work) and many of the passive devices (i.e., those that do not perform work) require a source of power, inductively coupled transcutaneous energy transfer (TET) and information transmission systems for such devices are coming into increasing use. These systems consist of an external primary coil and an implanted secondary coil separated by an intervening layer of tissue.
One problem encountered in such TET systems is that the best place to locate control circuitry for converting, amplifying and otherwise processing the signal received at the secondary coil before sending the signal on to the utilization equipment is within the secondary coil itself. However, there is also a significant magnetic field in the secondary coil resulting from the current induced therein, which field can induce heating of the components, particularly metallic components. At a minimum, such heating can influence the performance of various components, and in particular interfere with the desired uniform power applied to the equipment. In a worst case, the heating can be severe enough to cause damage or destruction to the components which can only be repaired or replaced through an invasive surgical procedure. Such heating can also cause injury or discomfort to the patient in which the components have been implanted.
Heretofore, in order to avoid such heating, it has either been necessary to be sure that the signal induced in the secondary coil is not sufficient to generate a magnetic field which would cause potentially damaging heating of the components or to mount the components at a less convenient location. The former is undesirable because it is generally not possible to eliminate significant heating of the components while still operating the device at required energy levels, and the later solution is not desirable since the output signal from the secondary coil can reach 500 volts and above at an operating frequency that can be in excess of 100 kHz. It is preferable that such high voltage signal not pass extensively through the body and it is difficult to provide good hermetically sealed connectors for signals at these voltages. It is therefore preferable that an auxiliary signal processing module, which may reduce the voltage to a value in the approximately 20 volt range, be included as close to the secondary coil as possible, a position inside the secondary coil being ideal for this purpose.
A need therefore exists for an improved technique for use with TET devices so as to enable at least selected electronic components to be mounted within the secondary coil with minimal heating of such devices.